Custom autosound sells a dual cone 4x10 speaker.
I bought one several years ago through Classic Industries - and just (re)confirmed that it is 4ohm.
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1970 Formula 400 Carousel Red on black (std) interior "no drivetrain option" car (same base drivetrain as GTO) 1:411 1970 Firebird Formulas originally sold in Canada
I actually have 2 new, "2008", dual cones but they are bracket type.
The info at, Turnswitch.com, (see above address), says you can damage the old radio using a 4 ohm speaker with an original radio. I know noting about electronics or speakers but interesting reading and trying to understand.
Good news. "Turnswitch,com" have a Canadian dealer here in Vancouver.
Great guys to talk to and deal with. Seems to be the answer to speaker questions and supply. They are temporarily out of stock until next week but they have what I need, "slimline", who knew there were so many different underdash speakers and that our Beaumonts used different speakers than a Chevelle.
Forgot to forward, "Woodradios.com", is the website and located in Stave Falls, for those not around here it is just outside Vancouver near Mission BC.